1. Field of the Invention
Epoxides having more than one epoxy group on average in the molecule, when combined and mixed with various curing agents, may be cured at an ambient temperature or an elevated temperature. Since the cured epoxides exhibit excellent adhesive properties to various materials, they are used in a variety of fields of application, such as adhesives, coatings and impregnants, and in injection, casting and potting. The present invention is intended to provide, in the fields relating to epoxides, epoxide compositions which can be easily handled and have excellent curing characteristics. More specifically, the present invention is intended to provide one-component heat curable epoxide compositions which have an excellent storage stability and can be cured in a short period of time at a relatively low temperature, and in particular, have a property of generating outgas in a smaller amount, and providing cured product with an excellent heat resistance.
2. Description
Most of the conventionally used epoxide compositions are of two-components, and must be used immediately after mixing an epoxide and a curing agent or a curing accelerator. Although two-component epoxide compositions may be cured at room temperature or at a low temperature, they must be precisely weighed and mixed shortly before used. If wrong amounts are weighed relative to the correct amounts of the epoxide and the curing agent, or the subsequent mixing is insufficient even if the correct amounts are weighed, a problem arises in that the properties of the cured product are insufficient. Further, almost all of conventional two-component epoxide compositions have a limitation in applications thereof because of a short pot life, and thus have a disadvantage in that they are difficult to be applied to automated machines. Thus, it is desired to develop one-component heat curable epoxide compositions, to remedy these disadvantages.
One-component heat curable epoxide compositions need a latent-type curing agent which has a property of not reacting with an epoxide at room temperature but of starting a curing reaction upon heating. Some latent-type curing compounds have been proposed, and typically there may be mentioned dicyandiamide, dibasic acid dihydrazides, boron trifluoride amine complex salts, guanamines, melamine and imidazoles. Although mixtures of dicyandiamide, melamine or a guanamine with an epoxide have an excellent storage stability, they have a disadvantage of necessitating a curing condition at a high temperature above 150° C. for a long period of time. Although it is well known to shorten the curing time by the use of a curing accelerator, the addition of the curing accelerator has a disadvantage of an extremely low storage stability. Also, dibasic acid dihydrazides and imidazoles have a poor storage stability even though they cure the epoxide at a relatively low temperature. Boron trifluoride amine complex salts have the advantages of an excellent storage stability and short curing time. But the cured products have a poor resistance to water and cause a corrosion of metals. Thus, each of these curing agents has a disadvantage. Under such circumstances, it is desired to develop an epoxide composition which has an excellent storage stability, can be cured at a low temperature in a short period of time, and exhibits good properties when cured.
To remedy the above disadvantages, Japanese Unexamined Patent Publication No. 3-296525 discloses that a compound, which is prepared by reacting with heating (a) N,N-dialkylaminoalkylamine, (b) an amine having one or two nitrogen atoms with active hydrogen and having a cyclic structure in the molecule thereof, (c) diisocyanate, and (d) an epoxide having more than one epoxy group in the molecule thereof, or which is prepared by reacting with heating the N,N-dialkylaminoalkylamine (a), the amine (b), and the diisocyanate (c), is useful as a latent-type curing agent. The epoxide composition containing this curing agent, however, produces a gas resulting from a residual solvent and low-molecular-weight component, a so-called outgas, when heated. In addition, the heat resistance of the cured product is poor. Japanese Unexamined Patent Publication No. 3-177418 discloses that a compound prepared by reacting with heating (a) a diamine compound having two amino groups, (b) N,N-dialkylaminoalkylamine, (c) an epoxide having more than one epoxy group on average in the molecule thereof, (d)urea, and if necessary (e) a monoamine compound, is useful as a latent-type curing agent. However, the epoxide composition containing the curing agent also produces an outgas from the low-molecular-weight component when heated. In addition, the heat resistance of the cured product is poor. Japanese Unexamined Patent Publication No. 6-184274 discloses that a compound prepared by reacting with heating (a) aminoalkylimidazole (b) an amine having one or two nitrogen atoms with active hydrogen and having a cyclic structure in the molecule thereof, (c) diisocyanate, and (d) an epoxide having more than one epoxy group on average in the molecule, is useful as a latent-type curing agent, and the resulting cured product has an excellent heat resistance. However, the epoxide composition containing the curing agent also produces an outgas from residual solvent and low-molecular-weight component when heated.